The invention concerns a cryostat configuration, comprising an outer shell and a cryocontainer for cryogenic fluid installed therein, wherein the cryocontainer is connected to the outer shell via at least two suspension tubes, and with a neck tube whose warm upper end is connected to the outer shell and whose cold lower end is connected to the cryocontainer, and which contains a cold head of a multi-stage cryocooler, wherein the outer shell, the cryocontainer, the suspension tubes and the neck tube delimit an evacuated space, wherein the cryocontainer is surrounded by at least one radiation shield which is connected in a thermally conducting fashion to the suspension tubes-and optionally also to the neck tube of the cryocontainer.
A cryostat configuration of this type is disclosed e.g. in U.S. 2002/0002830.
In cryostat configurations of the type, used e.g. in nuclear magnetic resonance (NMR) apparatus, a superconducting magnet coil system is disposed in a first container having a cryogenic liquid, usually liquid helium, which is surrounded by radiation shields, super insulation foils and optionally a further container with cryogenic liquid, usually liquid nitrogen. The liquid containers, radiation shields and super insulation foils are accommodated in an outer container which delimits a vacuum chamber. The superconducting magnet is kept at a constant temperature by the surrounding evaporating helium. The elements surrounding the helium container thermally insulate the container to minimize heat input into the container as well as the helium evaporation rate.
Magnet systems for high-resolution NMR spectroscopy are usually so-called vertical systems, wherein the coil configuration axis and the opening for receiving the NMR sample extend in the vertical direction. The helium container of NMR spectrometers is usually connected to the outer vacuum sleeve via at least two thin-walled suspension tubes. The container is thereby mechanically fixed and the suspension tubes provide access to the magnet as is required e.g. for charging. The waste gas is discharged via the suspension tubes thereby cooling the suspension tubes and, in the ideal case, completely compensating for the heat input via the tube wall. A system of this type is described e.g. in DE 29 06 060 A1 and in the document “Superconducting NMR Magnet Design” (Concepts in Magnetic Resonance, 1993, 6, 255-273).
Mechanical cooling apparatus, so-called cryocoolers, have recently been used to a greater extent for directly cooling superconducting magnet systems. In addition to cooling without cryogenic fluids (dry cooling) there are conventional systems which contain at least one further cryogenic fluid which is, however, reliquefied by the cryocooler after evaporation. For this reason, none or nearly none of the cryogenic fluid escapes to the outside. The documents EP 0 905 436, EP 0 905 524, WO 03/036207, WO 03/036190, U.S. Pat. No. 5,966,944, U.S. Pat. No. 5,563,566, U.S. Pat. No. 5,613,367, U.S. Pat. No. 5,782,095, US 2002/0002830 and US 2003/230089 describe such possible cooling of a superconducting magnet system using a cryocooler without losing cryogen.
The e.g. two-stage cold head of the cryocooler may be installed in a separate vacuum space (as described e.g. in U.S. Pat. No. 5,613,367) or directly in the vacuum space of the cryostat (as described e.g. in U.S. Pat. No. 5,563,566) in such a manner that the first cold stage of the cold head is fixed to a radiation shield and the second cold stage is connected in a thermally conducting fashion to the helium container either directly or indirectly via a fixed thermal bridge. The overall heat input into the helium container can be compensated for through back condensation of the helium, which evaporates due to heat input from the outside, on the cold contact surface in the helium container, permitting loss-free operation of the system. Disadvantageously, the connection between the second cold stage and the helium container has a thermal resistance.
One way to avoid this thermal resistance is to insert the cold head into a neck tube which connects the outer vacuum sleeve of the cryostat to the helium container and is correspondingly filled with helium gas as described e.g. in the document U.S. 2002/0002830. The first cold stage of the two-stage cold head is in fixed conducting contact with a radiation shield. The second cold stage is freely suspended in the helium atmosphere and directly liquefies the evaporated helium.
A superconducting magnet coil can become resistive, i.e. have a measurable electric resistance, e.g. due to slightly shifted wires in the coil packet, causing the overall magnet to quench. During a quench, the magnetic energy stored in the magnet is converted into heat and is suddenly released. In a magnet cooled by liquid helium, a considerable part or the whole amount of liquid thereby evaporates and must be discharged from the first tank to prevent generation of an inadmissibly high pressure which could damage the container. In a conventional cryostat, the quench gas is discharged via the suspension tubes of the helium container and through special safety valves, so-called quench valves, which present a large opening cross-section when the quench pressure is reached.
A quench and the associated sudden escape of the quench gas represent a problem, in particular, if the cooler of a cryogen-loss-free magnet system must be replaced due to a technical defect. In a system with which the cooler is installed in a neck tube which directly connects the outer vacuum sleeve to the helium container, the charged magnet may quench during removal of the cooler, possibly causing injuries to the technician due to cold gases (in the range of approximately 5 K to 100 K) and the quench pressure. For this reason, the magnet must initially be discharged before removal of the cryocooler. After renewed charging of the magnet, it generally takes some time until the magnetic field is stable enough to acquire MR spectra which has the consequence that, in particular, large magnet systems cannot be operated for several days (“downtime”).
It is therefore the underlying purpose of the present invention to propose a cryostat configuration which permits safe installation and removal of the cold head without interrupting operation of the system contained in the cryostat configuration, e.g. a magnet system.